The invention relates to tires of the so-called "tubeless" type and, more particularly, to the beads of such tires.
Such tires, which are widely used, particularly for trucks, have a radial carcass reinforcement which is reinforced with cables located approximately in planes including the axis of rotation of the tire, surmounted by a crown reinforcement and a tread, the carcass ply or plies being wound at each of their two ends around a substantially unstretchable bead ring.
In the case of tubeless tires, it is necessary that the inflation atmosphere not be subjected to losses by diffusion into the materials of the mounted assembly or by leaks caused by a poor seal of the different connecting surfaces between tire and rim.
In order to minimize the diffusion phenomena as much as possible, the inner surface of these tires is generally covered with a layer of "inner rubber" selected for its excellent impermeability to the inflation atmosphere. In general, the vulcanized rubbers used for this internal layer do not have good mechanical properties since they are frequently formulated with rubbers having a base of polyisobutylene or their substantially non-unsaturated, slightly reinforced derivatives in order to optimize the properties of impermeability to the inflation atmosphere, resilience, deformability, and adhesion to the carcass. French patent application 72/43974 describes this aspect of the prior art.
In order to minimize the leaks possible at the contact interfaces of the beads on the rim, particular care must be taken in the design and production of these surfaces, as well as in maintaining them in good condition. Tightness is assured by good pressure contact of the beads of the tires on the rims, the clamping force being assured by the internal pressure of the inflation atmosphere and the shapes of the surfaces which rest against each other.
The contact between the bead of the tire and the rim must also assure good transmission of the mechanical forces resulting from the driving or braking torques by a connection which is without substantial slippage, avoiding excessively rapid wear of the surfaces of the bead. In order to fulfill this role, the vulcanized rubber touching the rim and forming the transition with the carcass reinforcement and the bead ring or rings has high values of hardness and modulus of elasticity. It is generally formulated with elastomers which are generally very unsaturated, highly loaded and cross-linked. This mix, which also assures a role of protecting the bead ring or rings and the carcass reinforcement or reinforcements is frequently known as a "protector", since it in particular prevents direct injury to them by the tools used during the operations of mounting and dismounting the tire on its rim.
Tire carcass reinforcements have experienced substantial improvements in quality which makes it possible to reuse them several times by retreading the portion which has become worn in contact with the ground. The surfaces and/or edges of the rubber of the beads are therefore subjected to more frequent mounting and dismounting operations, which can damage them. Furthermore, the beads are subjected to greater aging stresses, due to the longer operating life of the carcasses. In fact, after prolonged tests of tires mounted on rims subjected to heating due to braking, deterioration is noted in the adherence of the carcass reinforcement cables in the region of the beads, as well as a change in the properties of the protector in contact with the rims and of the part in contact with the inflation atmosphere; these parts of the protector are substantially hardened and may even thereby become much too fragile and brittle. This hardening and the cracking which results from it favor leakage of the inflation atmosphere.